1. Field of the Invention
The present invention relates generally to the fields of image transfer and the photolithographic transfer of images to a semiconductor wafer, printed circuit board or other substrate, and more particularly, to an apparatus and method for selectively exposing a photosensitive layer of material to a patterned source of light without the need for a mask.
2. Description of the Relevant Art
Image transfer of complex patterns onto substrates such as semiconductor wafers, printed circuit boards, flat panel displays, and the like commonly employ the use of a photolithographic apparatus containing a light source, a system of lenses and/or mirrors and a photomask, mask or reticle. In common step-and-repeat, or step-and-scan systems, light moves from a light source, through a lens/mirror assembly, and through a patterned mask onto the substrate which is covered with a photosensitive polymer resist. The mask is a two dimensional stencil of the pattern to be transferred. Often, thirty or more different patterns, or layers, are transferred to a given substrate during the manufacturing process, requiring a different mask for each such layer. The mask manufacturing process has proven to be a costly and time-consuming process. The mask itself is easily damaged during everyday handling; accordingly, it is common to produce duplicate masks in order to replace masks that become damaged. Elimination of the various masks would save time and money in the manufacturing process.
The advantages inherent in eliminating the need for such exposure masks has long been recognized, and those skilled in the art have explored mask free lithography in depth. Current direct-write methods are known, such as laser, electron or ion beam lithography, wherein a fine beam of light is selectively steered to shine on each point of the photosensitive film that needs to be exposed. For example, in U.S. Pat. No. 5,451,489 to Leedy, an electron beam is used to selectively expose a photoresist layer on a semiconductor wafer without the use of a mask. Similarly, in U.S. Pat. No. 5,109,149 to Leung, a laser beam is used in conjunction with a polygonal mirror, a beam expander, and a lens to selectively direct light onto the surface of a wafer mounted on an X-Y axis motorized table. However, these direct-write techniques have been proven to be too slow for economic commercial use.
In U.S. Pat. No. 5,691,541 to Ceglio, et al., a lithography system is described wherein a programmable array of xe2x80x9clight switchesxe2x80x9d, in the form of an array of digital micro-mirror devices, is provided to control the passage of light from a source to a photosensitive layer to be exposed. Each micro-mirror is either deflected through an angle to form a dark portion of the pattern, or undeflected to form a bright portion of the pattern. However, the device described by Ceglio, et al. is dependent upon the proper alignment of many small reflecting mirrors in order to reflect the desired image to the substrate. Reflective aberrations and mirror mis-alignment cause inaccuracies in the image that is projected onto the substrate.
In U.S. Pat. No. 5,781,331 to Carr, et al., an optical micro-shutter array is described that can be produced using known semiconductor fabrication processes. The disclosed optical shutter includes an aperture plate positioned in a light path, the aperture plate having an array of apertures formed therein. A series of microcantilevers are used to selectively cover the array of apertures, each microcantilever being associated with one of the apertures. Carr, et al. describe such microcantilevers as preferably being formed of two layers of material having different thermal coefficients of expansion, and preferably being thermally-actuated, although Carr, et al. also state that piezoelectric and electrostatic-originating forces may also be employed. When a microcantilever is heated by passing an electrical current through an associated resistor, the microcantilever curls away from the associated aperture, thereby allowing light to pass through such aperture. One disadvantage of using such microcantilevers is that their up and down curling/flexing motion tends to cause unwanted interference between two adjacent microcantilevers; if a first microcantilever is trying to curl up away from its aperture, and a second adjacent microcantilever is trying to move down toward its adjacent aperture, then the two microcantilevers may contact each other.
U.S. Pat. No. 5,808,384 to Tabat describes a micromechanical actuator that can be formed on substrates using lithographic processing techniques. Tabat describes such devices as being useful for, among other things, forming optical switches. A plunger having two magnetic heads is supported within a gap of a magnetic core to which an electrical coil is coupled. A pair of springs bias the plunger to a central position. The application of electrical current to the electrical coil moves the plunger back and forth in a linear movement depending upon the direction of current flow. However, the necessity of having the actuators pass through the core of the electrical coil places restrictions on how close two or more of such actuators can be positioned relative to each other. Moreover, the need to form spring-like biasing members within the substrate further complicates the fabrication of such devices.
Accordingly, it is an object of the present invention to provide a maskless photoresist exposure system which eliminates the need for masks in order to selectively expose photosensitive layers applied to semiconductor wafers, printed circuit boards, or other substrates.
Another object of the present invention is to provide such a maskless photoresist exposure system capable of using a conventional photolithographic light source and avoiding the need for lasers, electron beams, or ion beams.
Still another object of the present invention is to provide such a maskless photoresist exposure system which operates quickly enough to prove economically feasible for commercial use.
A further object of the present invention is to provide such a maskless photoresist exposure system which avoids the need for precise alignment of small mirrors in order to produce a patterned light image.
A still further object of the present invention is to provide such a maskless photoresist exposure system which includes a series of optical shutters that can be disposed closely proximate one another to form adjacent pixels of light, yet wherein movement of one such shutter does not interfere with movement of shutters adjacent thereto.
A yet further object of the present invention is to provide such a maskless photoresist exposure system wherein the aforementioned series of optical shutters can themselves be formed using known photolithographic semiconductor processing techniques.
Yet another object of the present invention is to provide such a maskless photoresist exposure system wherein the aforementioned series of optical shutters need not themselves pass through the core of an electrical coil.
Still another object of the present invention is to provide a method of performing mask free photolithography.
An additional object of the present invention is to provide a novel method of forming an electrical coil, suitable for use in forming an electromechanical actuator, on a semiconductor substrate using known semiconductor wafer processing techniques.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.
Briefly described, and in accordance with the preferred embodiment thereof, the present invention relates to a maskless exposure system for selectively exposing a photosensitive work surface of a work material to light. Such photosensitive work surface could be a photosensitive film applied over the work material, such as a photoresist layer; alternatively, the work material may itself be photosensitive, such as photo-imagable glass, in which case the photosensitive work surface is merely an outer surface of the work material. The maskless exposure system of the present invention includes a generally planar substrate having an elongated aperture formed therein. A light beam from a source of light having a desired wavelength is directed generally toward the elongated aperture of the aforementioned substrate. The aforementioned source of light may include a projection system of lenses and/or mirrors to focus the beam of light toward the elongated aperture of the substrate. A plurality of shutter elements are formed upon the substrate generally parallel thereto and movably supported thereon. Each of the shutter elements is adapted to move between a first position covering a portion of the elongated aperture, and thereby preventing the passage of light through such portion, and a second position exposing a portion of the elongated aperture for allowing the passage of light therethrough. In order to control movement of the various shutter elements, a series of actuators are also included; each actuator cooperates with one of the shutter elements for selectively moving it between its first and second positions. These actuators are preferably computer-controlled so that desired movements of the shutter elements can be programmed. The combination of the covered and exposed portions of the elongated aperture creates a patterned light beam exiting the elongated aperture of said substrate.
The material bearing the photosensitive work surface is supported by a holder. A stepper device is provided for adjusting the relationship between the patterned light beam exiting the elongated aperture of the substrate and the work material holder in order to direct the patterned light beam at different portions of the work material. This stepper is again preferably controlled by a computer for programming movements of the patterned light beam relative to the work material, and to synchronize the operation of the shutter elements with the operation of the stepper. In one embodiment of the present invention, the stepper is a scanning mirror which is disposed at various angles to reflect the patterned light beam onto different portions of the work material; in an alternate embodiment, the patterned light beam is aimed directly onto the work material, and the stepper is an indexing system for physically shifting the work material holder until all portions of the work material have been exposed to the patterned beam.
Preferably, the aforementioned substrate in which the aperture is formed is constructed from a semiconductor material such as silicon, and the series of shutter elements are also formed of semiconductor material, using conventional semiconductor wafer processing techniques. In this regard, the semiconductor material forming such shutter elements can be semiconductor material that was initially deposited upon the substrate using known chemical vapor deposition techniques.
Ideally, each of the shutter elements slides in a linear motion between its first position covering a portion of the aperture, and its second position exposing such portion of the aperture. To facilitate such sliding motion, guides can be formed upon the substrate. Such guides can be disposed below and/or extend around each shutter element for guiding the movement thereof.
In those instances wherein two adjacent shutter elements both assume their first positions for blocking the passage of light through the aperture, it is desired that no light gap exist between the two adjacent shutter elements. Accordingly, abutting side edges of two adjacent shutter elements preferably extend closely proximate one another to avoid creation of any light gaps therebetween. In order to help position the abutting side edges of adjacent shutter elements as close together as possible, Applicant has found it to be helpful to position successive shutter elements on opposite sides of the elongated aperture.
In the preferred embodiment of the present invention, the shutter elements are actuated electromagnetically. In this regard, each shutter element includes a small permanent magnet affixed to an end of the shutter element, and each actuator includes an electromagnet in the form of a coiled electrical conductor for repelling or attracting the permanent magnet formed upon the related shutter element. The electrical coil extends along a longitudinal axis that is generally parallel to, and coaxial with, the axis of linear movement of the shutter element. To prevent one such coil from influencing a neighboring shutter element, magnetic shielding is preferably interposed between adjacent actuators.
The present invention also relates to Applicant""s method for selectively exposing a light pattern to a photosensitive work surface of a work material. In practicing such method, a generally planar substrate is provided having an elongated aperture formed therein. A beam of light is directed generally toward the elongated aperture of the substrate. A series of shutter elements are supported upon the substrate, each of the shutter elements being movable between a first position covering a portion of the elongated aperture and a second position exposing a portion of the elongated aperture. These shutter elements extend generally parallel to the plane of the substrate whether in the aforementioned first position or second position. Each of the shutter elements is moved to either its first position or its second position to selectively block or pass portions of the beam of light in order to create a patterned light beam that exits through the elongated aperture of the substrate; preferably, each of the shutter elements moves between its first and second positions by a sliding motion along a linear path. The resulting patterned light beam is then directed toward a first portion of the photosensitive work surface of the work material. After selectively exposing the first portion of the photosensitive work surface, the step of moving the shutter elements is repeated to create a new patterned light beam that is directed at a second portion of the photosensitive work surface of the work material. This process can be repeated until substantially all portions of the photosensitive work surface have selectively been exposed to an appropriately patterned beam of light.
Preferably, the method described above includes the step of forming the substrate from a wafer made of a semiconductor material. The step of forming the series of shutter elements preferably includes the step of forming a first sacrificial layer of a material upon a surface of the substrate; this first sacrificial layer is an oxide or nitride of the semiconductor material that forms the substrate. Next, a first layer of semiconductor material is deposited upon the first sacrificial layer, and this first layer is then patterned to form the shutter elements. The first sacrificial layer underlying the patterned deposited first layer of semiconductor material is then etched to allow the shutter elements to move relative to the substrate.
If desired, the method of the present invention may also include the step of forming a second sacrificial layer of material upon the surface of the substrate overlying the deposited first layer of semiconductor material; this second sacrificial layer may also be an oxide or nitride of the semiconductor material. A second layer of semiconductor material is then deposited upon the second sacrificial layer, and is patterned to form guide elements that bridge across the shutter elements. The second sacrificial layer that underlies the patterned deposited second layer of semiconductor material is then etched away for allowing the shutter elements to slide within the guide elements. Each shutter element is preferably positioned within at least one surrounding guide for guiding sliding movements of each such shutter element. Ideally, successive shutter elements are positioned on alternating sides of the elongated aperture.
In order to move the shutter elements, the method of the present invention preferably includes the steps of providing a small permanent magnet on each shutter element, forming an electromagnet, including an electrical coil through which electrical current can flow, proximate each shutter element, and controlling the direction of current flow through each such electrical coil in order to repel or attract each of the shutter elements.
The electrical coils described in the preceding paragraph can advantageously be formed by the method of depositing a first layer of metal upon the semiconductor substrate, and patterning the deposited first layer of metal to form a number of lower rungs of the electrical coil. A first insulative layer of material is then formed over the substrate and above the patterned deposited first layer of metal. A layer of magnetic material is then formed above the first insulative layer and extending across the plurality of lower rungs of the electrical coil. A second insulative layer of material is formed over the substrate above both the first insulative layer and the layer of magnetic material. Holes are then etched through the first and second insulative layers above the opposing first and second ends of the lower rungs to expose the ends of such lower rungs. A second metal layer is deposited upon the substrate over the second insulative layer and within the etched holes that expose the ends of the lower rungs. The second metal layer is patterned to connect the second end of each lower rung to the first end of a next succeeding lower rung to form the electrical coil. A third insulative layer of material is then formed over the substrate, extending above the electrical coils to insulate the second metal layer portion of the electrical coils. Preferably, magnetic shielding material is deposited between, but spaced apart from, adjacent electrical coils to isolate the electromagnetic field in one coil from adjacent coils.